Actinomyces is a genus of the actinobacteria class of bacteria. They are all Gram-positive. Actinomyces are facultatively anaerobic (except A. meyeri, a strict anaerobe). All species grow best under anaerobic conditions. Actinomyces species do not form endospores, and, while individual bacteria are rod-shaped, Actinomyces colonies form fungus-like branched networks of hyphae. The aspect of these colonies initially led to the incorrect assumption that the organism was a fungus and to the name "Actinomyces" or ray fungus.
link to wiki